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We employ the three-dimensional magnetohydrodynamic simulation includingambipolar diffusion to study the gravitationally-driven fragmentation ofsubcritical molecular clouds, in which the gravitational fragmentation isstabilized as long as magnetic flux-freezing applies. The simulations show thatthe cores in an initially subcritical cloud generally develop gradually over anambipolar diffusion time, which is about a few $\times 10^7$ years in a typicalmolecular cloud. On the other hand, the formation of collapsing cores insubcritical clouds is accelerated by supersonic nonlinear flows. Our parameterstudy demonstrates that core formation occurs faster as the strength of theinitial flow speed in the cloud increases. We found that the core formationtime is roughly proportional to the inverse of the square root of the enhanceddensity created by the supersonic nonlinear flows. The density dependence issimilar to that derived in quasistatically contracting magnetically supportedclouds, although the core formation conditions are created by the nonlinearflows in our simulations. We have also found that the accelerated formationtime is not strongly dependent on the initial strength of the magnetic field ifthe cloud is highly subcritical. Our simulation shows that the core formationtime in our model subcritical clouds is several $\times 10^6$ years, due to thepresence of large-scale supersonic flows ($\sim 3$ times sound speed). Once acollapsing core forms, the density, velocity, and magnetic field structure ofthe core does not strongly depend on the initial strength of the velocityfluctuation. The infall velocities of the cores are subsonic and the magneticfield lines show weak hourglass shapes.

Formation of collapsing cores in subcritical magnetic clouds: three-dimensional MHD simulations with ambipolar diffusion